This invention relates to elongated weak-link supercurrent devices and, more particularly, to controlling the propagation of mobile flux vortices in such devices.
In U.S. Pat. No. 3,676,718 granted to P. W. Anderson, R. C. Dynes and T. A. Fulton on July 11, 1972, there are described a variety of weak-link supercurrent devices (e.g., shift registers, memories) which are capable of sustaining one or more trapped magnetic field (flux) vortices which correspond to information bits. In an extended Josephson junction (SIS) device, that is, one which is long in the x-direction compared to the Josephson penetration depth .lambda..sub.J, the patent teaches that a vortex is induced by a spatial variation of the supercurrent J in which a positive supercurrent flows through the I-layer and into the contiguous superconductor to a depth of about .lambda..sub.L, the London penetration depth, then along the superconductor a distance of about 2.lambda..sub.J, thence through the I-layer again as a negative supercurrent into the opposite superconductor to a depth of about .lambda..sub.L and finally back to the point of beginning. Such a vortex supports a net magnetic flux of approximately .PHI..sub.o = 2.07 .times. 10.sup..sup.-15 Wb, the well-known flux quantum. As defined in the patent, the term vortex means an entity which includes both the circulating supercurrent J and the flux quantum .PHI..sub.o induced thereby.
Once created, the patent states, a vortex prefers to position and distribute itself in a region so that a local minimum of the sum of the magnetic energy plus the Josephson coupling energy is established. These local minima can be thought of as magnetic potential wells in which vortices can be stored. Where a plurality of such potential wells are present in a single weak-link structure, it is possible to move the vortex from one well to another by applying a force thereto as, for example, by applying a local current or magnetic field to a region near to the vortex. As discussed at column 4, lines 14-41 of the patent, potential wells can be created in a number of ways including periodically varying the thickness of the weak-link layer in an SIS structure (FIG. 6 of the patent), using point sources of magnetic field (FIGS. 7A and 7B of the patent), using locally applied currents at periodic locations of the structure (FIG. 8 of the patent), periodically varying the self-inductance per unit length of the structure (FIG. 9 of the patent) or, alternatively, varying the thickness of at least one of the superconductors so that in each memory location it is less about 2.lambda..sub.L.
For some applications, however, it may be inconvenient to form the potential wells by using a pair of wires at each well location or point sources of magnetic field. Moreover, having to vary oxide or superconductor thickness complicates processing and may reduce device yield.